Moist air will cause oxidation and corrosion to metal materials such as tinplate, which will lead to rust, looseness, delamination, pulverization, etc. on the metal surface, and make the metal surface lose its original characteristics and luster. The critical value of air relative humidity for metal oxidation corrosion is 70% for steel, 63% for iron, 60% for zinc and 76% for aluminum. In humid air above the critical value, the oxidation, corrosion and rust of metals increase with the increase of humidity.

When the air humidity reaches about 85%, since the iron base has been exposed at the notch on both sides of the sheet after slitting, the sheet directly exposed to the air or not tightly packaged will show rust within 3 days. When the air humidity reaches about 90%, the tinplate directly exposed to the air will show rust in three hours and become very serious in five hours.